This invention relates to an improved contrast enhancement glass useful for ophthalmic applications such as sunglass lenses, and having desirable and/or necessary properties related to appearance, compatibility with visibility and sight discrimination requirements such as for traffic signal transmittance, etc.
Thus, it is desired for many applications to enhance the contrast of target subjects with respect to background radiation. In addition, it is often desired to lower the amount of UV radiation transmitted through a given glass, e.g., for sunglasses, preferably to obtain 0% transmission up to a wavelength of 380 nm and in some cases, e.g., where UV radiation is especially harmful, up to and/or around 400 nm. While it has been possible to obtain either of these effects alone and, to a certain extent, both effects together, there still remains room for improvement, especially in glasses which are brown, especially while simultaneously meeting stringent requirements such as for traffic light transmittance. Of course, it is also desirable that, for ophthalmic and/or optical applications, the glass be capable of providing severe lens corrections (high index) and have a low density.
It is known that Nd.sub.2 O.sub.3 can provide contrast enhancement due to its natural combination of absorption bands in the visible region. See, e.g., U.S. Pat. Nos. 4,769,347, 4,288,250, 4,521,524, 4,376,829, 4,405,881 and 4,390,637. In contrast enhancement glasses containing Nd, it is also desired that the radiation coming from a desired subject maintain the true color of the latter on passage through the glass and that the image of the subject be sharp. With such properties, the glass would be most useful plano or in prescription lenses.
Neodymium has also been added to various glasses for other purposes. See, e.g., U.S. Pat. Nos. 1,449,793 (coloring), 2,688,560 (coloring), 3,457,182 (lasing), 3,640,890 (lasing) and 4,093,468 (coloring of glassceramics). In addition, it is known to add colorants such as CeO.sub.2, Sm.sub.2 O.sub.3, Fe.sub.2 O.sub.3, CoO, NiO, CuO, Er.sub.2 O.sub.3, MnO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.3, etc., for purposes of obtaining desired colors in silicate glasses, in various combinations. CeO.sub.2 is also well known as a solarization inhibitor. See, e.g., U.S. Pat. Nos. 4,257,811, 3,495,964, 3,672,919 and 3,960,754, as well as many of the patents mentioned above. The ability of some of these colorants, as well as TiO.sub.2, to absorb short wavelength radiation is also known in general. See, e.g., U.S. Pat. No. 4,521,524.
Some of the aforementioned problems have been solved for grey glasses in U.S. Pat. No. 5,190,896, which also reports initial failed attempts at solving such problems for brown glasses. See its Examples J, K and 22. As noted at column 5, line 56 to column 6, line 10, as well as in view of the properties listed for these three examples in columns 7 and 8, while a brown color was achieved, simultaneous achievement of desirable and/or necessary properties was not achieved.